A substantial percentage of customer originated calls are presently keyed by callers using a TT telephone. The use of TT signaling has proven to be a valuable adjunct to telephone service in that it improves and simplifies customer "dialing" from a human factors standpoint and enables calls to be established more rapidly than with dial pulse signaling.
Many present day switching systems, such as the direct progressive or step-by-step systems, are basically designed to establish switching connections in response to dial pulse signals. In order to enable such systems to operate with customer TT telephones, it was necessary to equip the systems with facilities for converting the customer keyed TT signals into dial pulses.
Typically, such facilities include a plurality of converter circuits for serving a larger plurality of TT customer stations. A converter customarily is connected to a calling station for returning dial tone and then receiving and verifying the validity of the customer keyed TT digits. The converter usually converts the TT digits and then generates the dial pulses required for operating the switching network to extend the call toward its destination.
Efficient and rapid call processing requires that the TT to DP converter facilities ascertain when a calling customer has finished keying of a called customer number. This function permits a converter to complete its outpulsing operations, release rapidly from call connections and become available for serving other calls.
Despite the many innovations which have occurred in TT to DP converter arrangements, a recognized problem in the prior art has been the need for complex and expensive circuitry to be duplicated in each of the TT to DP converters in order to ascertain when the last TT digit has been keyed by a caller.